A. The Circulatory System And The Nature Of Hemoglobin
The blood is the means for delivering nutrients to the tissues and removing waste products from the tissues for excretion. The blood is composed of plasma in which red blood cells (RBCs or erythrocytes), white blood cells (WBCs), and platelets are suspended. Red blood cells comprise approximately 99% of the cells in blood, and their principal function is the transport of oxygen to the tissues and the removal of carbon dioxide therefrom.
The left ventricle of the heart pumps the blood through the arteries and the smaller arterioles of the circulatory system. The blood then enters the capillaries, where the majority of the exchange of nutrients and cellular waste products occurs. (See, e.g., A. C. Guyton, Human Physiology And Mechanisms Of Disease (3rd ed.; W. B. Saunders Co., Philadelphia, Pa.), pp. 228-229 [1982]). Thereafter, the blood travels through the venules and veins in its return to the right atrium of the heart. Though the blood that returns to the heart is oxygen-poor compared to that which is pumped from the heart, in resting man the returning blood still contains about 75% of the original oxygen content.
The reversible oxygenation function (i.e., the delivery of oxygen and the removal of carbon dioxide) of RBCs is carried out by the protein hemoglobin. In mammals, hemoglobin has a molecular weight of approximately 68,000 and is composed of about 6% heme and 94% globin. In its native form, it contains two pairs of subunits (i.e., it is a tetramer), each containing a heme group and a globin polypeptide chain. In aqueous solution, hemoglobin is present in equilibrium between the tetrameric (MW 68,000) and dimeric forms (MW 34,000); outside of the RBC, the dimers are prematurely excreted by the kidney (plasma half-life of approximately two to four hours). Along with hemoglobin, RBCs contain stroma (the RBC membrane), which comprises proteins, cholesterol, and phospholipids.
B. Exogenous Blood Products
Due to the demand for blood products in hospitals and other settings, extensive research has been directed at the development of blood substitutes and plasma expanders. A blood substitute is a blood product that is capable of carrying and supplying oxygen to the tissues. Blood substitutes have a number of uses, including replacing blood lost during surgical procedures and following acute hemorrhage, and for resuscitation procedures following traumatic injury. Plasma expanders are blood products that are administered into the vascular system but are typically not capable of carrying oxygen. Plasma expanders can be used, for example, for replacing plasma lost from burns, to treat volume deficiency shock, and to effect hemodilution (for, e.g., the maintenance of normovolemia and to lower blood viscosity). Essentially, blood products can be used for these purposes or any purpose in which banked blood is currently administered to patients. (See, e.g., U.S. Pat. Nos. 4,001,401 to Bonson et al. and 4,061,736 to Morris et al., hereby incorporated by reference).
The current human blood supply is associated with several limitations that can be alleviated through the use of an exogenous blood product. To illustrate, the widespread availability of safe and effective blood substitutes would reduce the need for banked (allogeneic) blood. Moreover, such blood substitutes would allow the immediate infusion of a resuscitation solution following traumatic injury without regard to cross-matching (as is required for blood), thereby saving valuable time in resupplying oxygen to ischemic tissue. Likewise, blood substitutes can be administered to patients prior to surgery, allowing removal of autologous blood from the patients which could be returned later in the procedure, if needed, or after surgery. Thus, the use of exogenous blood products not only protects patients from exposure to non-autologous (allogeneic) blood, it conserves either autologous or allogeneic (banked, crossmatched) blood for its optimal use.
C. Limitations Of Current Blood Substitutes
Attempts to produce blood substitutes (sometimes referred to as "oxygen-carrying plasma expanders") have thus far produced products with marginal efficacy or whose manufacture is tedious and expensive, or both. Frequently, the cost of manufacturing such products is so high that it effectively precludes the widespread use of the products, particularly in those markets where the greatest need exists (e.g., emerging third-world economies).
The blood substitutes that have been developed previously are reviewed in various references (See e.g., Winslow, Robert M., "Hemoglobin-based Red Cell Substitutes," Johns Hopkins University Press, Baltimore [1992]). They can be grouped into the following three categories: i) perfluorocarbon-based emulsions, ii) liposome-encapsulated hemoglobin, and iii) modified cell-free hemoglobin. As discussed below, none has been entirely successful, though products comprising modified cell-free hemoglobin are thought to be the most promising. Perfluorochemical-based compositions dissolve oxygen as opposed to binding it as a chelate. In order to be used in biological systems, the perfluorochemical must be emulsified with a lipid, typically egg-yolk phospholipid. Though the perfluorocarbon emulsions are inexpensive to manufacture, they do not carry sufficient oxygen at clinically tolerated doses to be effective. Conversely, while liposome-encapsulated hemoglobin has been shown to be effective, it is far too costly for widespread use (See e.g., Winslow, supra).
Most of the blood substitute products in clinical trials today are based on modified hemoglobin. These products, frequently referred to as hemoglobin-based oxygen carriers (HBOCs), generally comprise a homogeneous aqueous solution of a chemically-modified hemoglobin, essentially free from other red cell residues (stroma). Although stroma-free human hemoglobin is the most common raw material for preparing a HBOC, other sources of hemoglobin have also been used. For example, hemoglobin can be obtained or derived from animal blood (e.g., bovine hemoglobin) or from bacteria or yeast or transgenic animals molecularly altered to produce a desired hemoglobin product. (See generally, Winslow, supra).
The chemical modification is generally one of intramolecular crosslinking and/or oligomerization to modify the hemoglobin such that its persistence in the circulation is prolonged relative to that of unmodified hemoglobin, and its oxygen binding properties are similar to those of blood. Intramolecular crosslinking chemically binds together subunits of the tetrameric hemoglobin unit to prevent the formation of dimers which, as previously indicated, are prematurely excreted. (See, e.g., U.S. Pat. No. 5,296,465 to Rausch el al., hereby incorporated by reference).
The high costs of manufacturing HBOC products have greatly limited their commercial viability. In addition, the present inventors have found that known HBOCs have a tendency to release excessive amounts of oxygen to the tissues at the arteriole walls rather than the capillaries; this can result in insufficient oxygen available for delivery by the HBOC to the tissues surrounding the capillaries. This is despite the fact that the initial loading of the HBOC with oxygen may be relatively high, even higher than that normally achieved with natural red blood cells.
What is needed is a blood product that is relatively inexpensive to manufacture and that delivers adequate amounts of oxygen to the tissues.